1. Field of the Invention
This invention relates to improvements in controlling the speed of direct current motors in disk drives and more specifically to improvements in controlling the speed of direct current motors used in disk drives which have moving heads, and more specifically to a feedforward circuit and method which precompensates the speed control loop circuit for changes in drag on the motor due to head seeks.
2. Description of the Relevant Art
The problem addressed by this invention is encountered in the disk drive industry where the accuracy of rotational speed is increasingly important. The trend in the industry has been to increase the storage capacity and the accuracy of the disk drive while reducing the weight and power consumption. Consequently, drive manufacturers have increased the density of data on a disk in a disk drive while decreasing the inertial and rotational mass of the disk drive; therefore, the need for controlling the speed of a drive to a high degree of accuracy while minimizing jitter is becoming of paramount importance, especially in high density small package disk drives such as those used in the portable computer market. An additional trend in the industry is to decrease the average seek time for the disk drive. The decrease in seek time is typically accomplished by simply increasing the acceleration and deceleration of the read/write heads (r/w heads).
One problem in small disk drives is the fact that the position of the head will affect the drag on the rotating media. It has been observed that the rotational drag created by the heads is approximately proportional to the radial distance the heads are from the center of rotation. Consequently, the rotating media will either speed up or slow down when a head seek occurs because the drag on the motor changes, and because the speed control loop is often too slow, due to its low bandwidth, to compensate for the changes in drag. This problem is exacerbated as head seek times are decreased since the change in drag will occur much quicker. Additionally, as the inertial and rotational mass of the disk drive continues to decrease, the rotational drag on the media becomes a more significant factor to overcome. Therefore, one problem in small hard drives is the fact that the position of the head will affect drag and therefore affect the speed regulation of the spindle motor.
Referring now to FIG. 1, a disk drive system according to the prior art is shown. FIG. 1 shows the pertinent part of a head positioning circuit 10, which includes a voice coil motor digital to analog converter (VCM DAC) 12, a transconductance amplifier 14, a voice coil motor (VCM) 16, and read/write heads 18. In operation, a head position signal is generated by the control unit of the disk drive (not shown) and applied to the VCM DAC 12 on line 11. The VCM DAC 12 converts the head position signal into an analog signal which amplifier 14 converts into current for the VCM 16, which is mechanically coupled to the heads 18 of the disk drive. The r/w heads 18 move responsive to the current in the VCM 16, as is known in the art. The operation of head positioning circuits are further discussed in U.S. Pat. No. 5,297,024 and is fully incorporated hereinto by reference.
FIG. 1 also shows a speed control loop 20 which controls the speed of the spindle motor 30. In operation, the speed control loop 20 maintains the speed of the spindle motor 30 by comparing a feedback signal 21 to a reference signal in the FLL or PLL detector 22. From the comparison, an error signal is fed to the charge pump 24 which integrates the error signal. The integrated error signal is filtered by filter 26 and then converted to current by amplifier 28. The output of amplifier 28 is used to drive the stator coils of spindle motor 30. If the spindle motor is running too slow, the FLL or PLL detector 22 will sense the error and increase the output of charge pump 24. This increased output is sensed by the amplifier 28 which increases the drive current to the stator windings of the spindle motor 30 so that the spindle motor 30 speeds up. Conversely, if the motor is running too fast, the FLL or PLL detector 22 senses the extra speed and decreases the output of the charge pump 24 appropriately. Consequently, amplifier 28 will reduce the current to the stator windings of the spindle motor which will slow the motor. The method and apparatus for controlling the speed of a spindle motor in a disk drive is discussed in U.S. Pat. No. 5,223,772 and U.S. Pat. No. 5,204,594 and U.S. Pat. No. 5,293,445 and U.S. Pat. No. 5,329,560 and are fully incorporated into this specification by reference.
As discussed above, the problem with the prior art circuit is that changes in the position of the heads can affect the rotational velocity of the spindle motor. It has been observed that the bandwidth of the speed control loop 20 is typically too slow to compensate for speed variations due to head seeks.